There is a multi-RAT user equipment (UE) having capability in which the multi-RAT UE can access two or more radio access technologies (RATs). In order to access a specific RAT, connection to a specific RAT can be established on the basis of a UE request, and data transmission/reception can be achieved on the basis of the UE request.
However, although the multi-RAT UE has the capability to access two or more RATs, the multi-RAT UE cannot simultaneously access multiple RATs. In other words, although a current UE has multi-RAT capability, the UE cannot simultaneously transmit and receive data through different RATs.
Since the aforementioned legacy multi-RAT technology does not require interworking between a wireless LAN and a cellular network, overall system efficiency is low. And, power transmittable to a UE is fixed. Yet, when a multi-RAT UE transmits data via a plurality of communication systems at the same time, it is necessary to control power. Hence, it is necessary for the multi-RAT UE to report a power headroom report to a network to control power in a situation that a plurality of the communication systems are interworking.
However, a method for a multi-RAT UE to report a power headroom report in a situation that a plurality of the communication systems are interworking has not been specifically proposed yet.